[pianotech] petrof grand breaking strings

[Mike Spalding]

A non-computerized colleague has asked me to post this question:  A 
3-year old Petrof grand, 5' 8", is experiencing string breakage in the 
capo section, which in this piano extends from the top of the agraffed 
tenor all the way to note 88.  It is played several hours per day by a 
gifted 12-year-old boy.  He is normal size for his age.  The tech has 
filed hammers and increased let-off, but it continues to break strings.  
He has considered rounding off the v-bar, which he has observed (with a 
mirror) to be quite narrow and sharp.  Question: is this model 
particularly prone to breaking strings?  How would you attack this problem?

thanks